In 1929, German Admiral Wolfgang Wegener argued in The Sea Strategy of the World War that in the event of future conflict with Great Britain, Germany should seize Norway, the Shetland and Faroe Islands, and Iceland. Only thus could the German fleet escape the confines of the North Sea and gain a dominant strategic position in the North Atlantic. These arguments deeply influenced the leadership of the German navy. Admiral Erich Raeder, chief of the German navy, repeatedly pressed these views on Adolf Hitler.

At the outset of the war, however, Hitler supported neutrality in Scandinavia. This would ensure a continued flow of Swedish iron ore to Germany. The Swedish ores had an iron content of 60 percent, permitting great economies in the production of steel. Low in phosphorus, Swedish ore produced the best steel. Between December and April when the Baltic was frozen, iron ore from Swedish mines was sent by rail to the ice-free Norwegian port of Narvik and thence through neutral Norwegian coastal waters to German ports.

First Lord of the Admiralty Winston L. S. Churchill was keenly aware of the importance of this ore traffic and determined to halt it. The British and French planned an expedition to aid the Finns when the Soviet Union attacked Finland in November 1939, but in reality this plan was a cover for the Allied seizure of Narvik and the ore fields in northern Sweden. The end of the war in Finland compelled the Allies to abandon this plan. Churchill now sought to mine Norwegian waters, forcing German ore ships to sea where they could be taken. This violation of Norwegian neutrality would undoubtedly provoke a German reaction, allowing the British to land troops in Norway.

Convinced that the Allies were planning a Norwegian intervention, Hitler began in February 1940 to prepare for the German occupation of Denmark and Norway, Operation weserübung ( weser exercise). The invasion was scheduled for 20 March, but ice in the Baltic delayed it until 9 April. Meanwhile, the British began their minelaying operation in Norwegian territorial waters early on 8 April.

Churchill and the British were surprised by the magnitude of the German operation. For the assault on Norway, Hitler committed 5 infantry divisions, 2 regiments of mountain troops, and a battalion of light tanks: 66 PzKpfw Is and Iis. Operation weserübung would also see the first use of paratroops (3 companies) in combat. To seize 6 Norwegian coastal cities, the German navy transported 8,850 soldiers in 2 battleships, a pocket battleship, 2 heavy cruisers, 3 light cruisers, 14 destroyers, and smaller vessels. The remaining 54,500 troops would be sent in once these objectives were secure.

The Luftwaffe amassed more than 1,000 aircraft for the operation. Fast, heavily armed twin-engine Me-110 fighters possessed the range required to operate in this theater. Flying from captured airfields, Stuka dive-bombers proved devastatingly accurate. The most daring feature of German air operations was the first large-scale use of transport aircraft to deploy troops, support units, and supplies into the attack. More than 500 took part.

Norwegian defenses were woefully deficient. Of 63 warships in the Norwegian navy, only 4 small destroyers and a minelayer were modern. All but 10 of the 150 Norwegian warplanes were obsolete biplanes. (On order were 152 modern warplanes, 120 of them from the United States.) The Norwegian army lacked sufficient numbers of heavy machine guns and artillery, and it possessed no antitank guns. Much of the Norwegian population lived in coastal cities where mobilization centers and arsenals were located. Surprise German attacks captured much Norwegian equipment and disrupted Norwegian mobilization.

German preparations for the invasion were noted by British and Norwegian authorities. The British misread both the timing and the scale of the German attack, and Norwegian government attention was riveted on the British violation of Norway's neutrality. Thus the Norwegian Parliament authorized mobilization of the navy and air services but only 7,000 soldiers.

The Germans secured most of their immediate objectives. German paratroopers seized Sola, the largest Norwegian air base, and by 11 April, 200 German warplanes, especially Stukas, were flying from the captured airfield. The fortress of Oscarsborg commanded the water approach to Oslo. It mounted 3 11-in. guns, installed in 1905, and a torpedo battery. Scorning the weaponry as antiquated, the Germans assumed Oscarsborg was no longer manned. Firing at point-blank range and using torpedoes, the Norwegians sank the German heavy cruiser Blücher with heavy loss of life, and the German convoy it had been escorting was forced to withdraw. The Germans then took Oslo's Fornebu airport, and transport planes landed two battalions of troops who soon occupied the Norwegian capital. But King Haakon VII and his family, the government, and the parliament had already left Oslo.

The British navy struck swiftly at the German fleet. On 10 April, British destroyers attacked German destroyers at Narvik. Backed by the old battleship Warspite, the British attacked again on 13 April, sinking the last of the 10 German destroyers. But British warships operating off the central Norwegian coast without fighter protection came under attack from approximately 90 Luftwaffe bombers. The Royal navy quickly decided not to risk major warships within range of German air power.

German army commander General Nikolaus von Falkenhorst organized his forces in highly mobile groups, effectively supported by light tanks as ski troops from Bavaria and Austria repeatedly outflanked enemy defensive positions. Norwegian army commander General Otto Ruge sought to delay the German advance until Allied forces could arrive. Norwegian troops, poorly trained and armed, were in chaotic disorder and came under heavy air attack. But Ruge rallied his men and fought tenaciously in the narrow mountain valleys of central Norway. Such delaying actions made Allied expeditions to Norway possible.

The Allies committed 12,500 soldiers to central Norway. By 20 April, three battalions of British and three battalions of French mountain troops were at Namsos 127 miles northeast of Trondheim, and two British battalions (later reinforced by three additional battalions) were at Åndalsnes, 100 miles southwest of the major Norwegian city and transport hub. In a pincers movement, the two forces were intended to recapture Trondheim. But confusion in unit deployment led to chaos in logistical support in Britain. Allied troops arrived without necessary items, from ski bindings to artillery. The swift advance of the Germans forced the British at Åndalsnes to turn south and wage a series of defensive battles. Meanwhile, a Luftwaffe airlift reinforced the garrison in Trondheim. Allied bases at Namsos and Åndalsnes were reduced to ashes by German bombing. To the disappointment of the Norwegians, the Allies abandoned central Norway by 3 May.

Isolated at Narvik, the three battalions of German mountain infantry were supplied by Ju-52 transports and seaplanes flying at maximum range. On 15 April, a British guards brigade that included Scottish, Irish, and Welsh battalions arrived at Harstad, 70 miles from Narvik. Three battalions of French mountain infantry arrived later. But conflicting orders and clashing command personalities deadlocked Allied operations in the north.

Beginning on 10 May, the Germans launched a massive attack against France, the Netherlands, and Belgium. Compelled to consolidate their forces, the Allies decided to seize Narvik, destroy its ore-loading facilities, and then evacuate the royal family and government of Norway and Allied troops.

Disputes between the British army and navy were put aside, and a new army commander, Lieutenant General Claude Auchinleck, arrived on 11 May. Reinforcements ably led by French Brigadier General Antoine-Marie Béthouart included four battalions of Free Polish infantry and two battalions of the French Foreign Legion. Norwegian troops held the mountains ringing the town. Six hundred local Norwegians improved the air base at Bardufoss, and 14 Gladiator biplanes and 16 modern Hurricanes successfully covered the seizure of Narvik and the subsequent evacuation of Allied forces. On 28 May, French, British, Polish, and Norwegian troops ousted the Germans from Narvik and drove them to the Swedish border. The evacuation of 25,000 Allied soldiers was completed by 8 June. A British cruiser transported the Norwegian monarch and government to Britain. General Ruge disbanded his forces, and hostilities ended in Norway on 10 June.

In fighting on land, the Germans lost 3,692 men killed and missing, the Norwegians 1,335, and the French and Poles 530. The English lost 1,896 dead, missing, and severely wounded. The Luftwaffe lost 242 aircraft; the British lost 112 aircraft. In fighting at sea, the British lost an aircraft carrier, 2 light cruisers, and 7 destroyers. The French and Poles each lost a destroyer. The Norwegian Campaign badly damaged the German surface navy; its losses included 1 heavy cruiser, 2 light cruisers, and 10 destroyers. It now had only 3 cruisers and 4 destroyers undamaged, although other damaged ships could be repaired.

After repairs at Narvik, the flow of Swedish iron ore—the main reason Germany conquered Norway—continued without interruption to the end of the war. The strategic advantage of Norwegian bases came into play as the war unfolded. Based in Norwegian fjords, the battleship Tirpitz pinned down much of the battleship strength of the Royal Navy. Convoys to Russia were decimated by the Luftwaffe flying from northern Norway and submarines. But Germany was never able to use Norway as a springboard to the Faroes, Shetlands, and Iceland. The British took the Faroes under their protection on 12 April and Iceland on 10 May. The keys to the North Atlantic remained firmly in Allied hands.

An unforeseen result of the Norwegian fiasco was the resignation of Prime Minister Neville Chamberlain on 10 May following the debacle in central Norway. Winston L. S. Churchill, ironically the architect of the disaster, then became prime minister. His leadership would be effectively employed in the difficult summer and fall of 1940. Based in London, indomitable King Haakon VII and his Free Norwegians continued the struggle to liberate their homeland.